The present disclosure relates generally to a crane assembly for moving items into and out of storage. More particularly, the present disclosure relates generally to an improvement overhead traveling crane for transporting watercraft in and out of dry storage.
The dry storage of watercraft has become increasingly popular over the years. This dry dock storage typically includes a stacked or vertical arrangement of watercraft that is lifted from the water's surface and placed in a stacked arrangement. For example, U.S. Pat. Nos. 6,007,288; 7,112,007; 7,367,747; 5,489,033; 3,786,942; 4,190,031; 6,162,003; U.S. Publication No. 2002/0176767 and PCT No. US 2007/4005540 and similar art disclosed and referenced in those applications and publications discuss dry watercraft storage.
In the conventional art, the lifting apparatus for this dry watercraft storage is typically a cradle device or support device suspended by either ropes or cables. The ropes and cables are collected in drums positioned near bridge girders, or horizontal supports, which move the cradle within the storage facility. Alternately, the lifting apparatus for this dry watercraft storage can be a forked arm type device can be attached to a mast that extends down from a central location on those bridge armatures and supports the boat when transferred between storage and the water. This support extending vertically down has typically been a centrally located mast that is rotatable about a trolley or a turntable usually positioned on the bridge supports of the crane structure.
These existing cranes that have a central mast can be described as stacker cranes. They are typically balanced only in an unloaded condition. This is due to the fact that the heavy hoist and mast are centered on the turntable in the center of rotation of the turntable. The mast hangs in a vertically plumb position. In addition, the footprint of the turntable is typically small in diameter and does not accommodate a wide range of loads, or types of boats, and the varying center of gravities of those loads.
For many watercraft, when loaded onto these conventional carriage or fork type lifting devices, the watercraft's center of gravity will fall outside the rotation of the support structure, or outside of the center of gravity of the overall turntable mast and lifting structure. This puts a large eccentric momentum force on the rotational bearing. This results in an unbalanced and unstable system that requires high maintenance and experiences many system failure problems. Additionally, frequently when this conventional system is loaded, the mast is pulled excessively out of plumb.
Conventional logic has been to compensate for this shortcoming by over designing the lift system so that the crane and mast can strong arm the load. In this situation, the mast and crane are made excessively heavy and in turn are expensive.
These conventional systems also put strain on the bridge girders if the boats are of sufficient size to pull the center of gravity of the loaded conventional stacker crane outside the footprint of the bridge girders. This can put undue strain on the bridge girders. Or alternately, it can temporarily disengage the opposite end of the stacker crane system from the bridge girder and potentially overturn the whole assembly. Again, the conventional wisdom has been to over engineer the systems such that the mass and size of the conventional stacker crane assembly greatly compensates for this movement in center of gravity.
What is needed then is a new watercraft stacker crane system.